The risk of unacceptable radiation (RT)-induced lung injury remains a significant limiting factor in the current treatment of the tumors involving the thoracic region. Despite advances in normal tissue radiobiology demonstrating that ionizing radiation triggers a cascade of genetic and molecular events, which lead to pulmonary injury, it is still unclear how a prolonged response to injury can be sustained for months to years after irradiation has ended. This deficiency in understanding of the mechanisms of RT-induced lung injury has hindered the development of appropriate interventional approaches to prevent this serious problem. The current proposal is based on our recent finding implicating hypoxia as an important contributing factor in the development of RT-induced pulmonary injury. We believe that hypoxia results from two factors: 1) increased oxygen consumption by activated macrophages with associated production of reactive oxygen species (ROS) and cytokines, and 2) decreased oxygen delivery to tissue due to vascular damage causing reduced perfusion. We hypothesize that hypoxia mediates a cycle of continuous, macrophage-associated production of ROS and expression/activation of profibroqenic and proanqioqenic cytokines. This process leads to l disre.qulation of angiogenesis, endothelial cell death, and collagen deposition which result in sustained hypoxia that I_erpetuates further pulmonary tissue damage and fibrosis, The goal of this study is to determine the I temporal onset of hypoxia after lung irradiation, and to define how hypoxia relates to macrophage activity (the production of ROS and cytokines) and vascular damage at different time points after irradiation. Lung hypoxia will be determined using the EF5 hypoxia marker. Macrophage activation will be assessed by immunohistochemistry. ROS will be detected using electron spin resonance (ESR) spectroscopy and spin trapping. A radionuclide perfusion assay will be used to assess pulmonary perfusion. After characterizing the relationship between hypoxia, macrophage activation and vascular damage following RT we will attempt to disrupt this injury cycle in two ways. First, ROS will be targeted directly with superoxide dismutase (SOD) mimetics. Second, ROS mediated injury will be targeted indirectly by inhibiting macrophage activity with gadolinium chloride (GdCI3). If successful, this project may lead directly to the development of clinically applicable strategies to reduce the risk of RT-induced lung injury in an attempt to permit delivery of higher doses of radiation to thoracic tumors without increasing the risk of pulmonary complications.